Knowledge of wellbore placement and surveying is useful for the development of subsurface oil & gas deposits, mining, and geothermal energy development. Accurate knowledge of the position of a wellbore at a measured depth, including inclination and azimuth, may be used to attain the geometric target location of, for example, an oil bearing formation of interest. Additionally, accurate relative placement of a wellbore to a geological zone or formation, or relative to one or more adjacent wellbores, may be useful or necessary for the production of hydrocarbons or geothermal energy, or to ensure that adjacent wellbores do not physically intersect each other.
Traditional wellbore survey techniques utilize sensors including north-finding or rate gyroscopes, magnetometers, and accelerometers to measure azimuth and inclination, with depth resulting from drillpipe depth or wireline depth measurements. With traditional wellbore survey techniques, the resultant positional uncertainty between two or more adjacent wellbores may be too large to determine the distance or direction (relative orientation) between the adjacent wellbores within a desired accuracy or statistical confidence interval. In some instances, magnetic ranging techniques may consist of estimating the distance, orientation, or both the distance and orientation of a wellbore or drilling equipment in that wellbore relative to other wellbores by measuring the magnetic field that is produced either passively from the adjacent wellbore's casing or drillpipe, or by measuring an actively generated magnetic field. In some instances, the use of magnetic ranging techniques may result in decreased relative positional uncertainty between adjacent wellbores compared to traditional wellbore survey techniques.
In splitter wells, two wellbores may share the same conductor pipe. Traditionally, in splitter wells, two smaller casings are installed within the same larger conductor. The smaller casings may be in proximity to each other and in certain cases, touching. It is desirable that an exit from one casing, such as, for instance, by drilling out of the shoe or setting a whipstock, does not result in a collision with the other casing. Because both wellbores are cased, the use of magnetic ranging techniques may result in inaccurate results.
When blind drilling, conductor pipes are driven, for instance, from offshore platforms; the position of the bores relative to each other may not be known or not known to a desired accuracy. It is desirable that the bores not intercept each other. Like in splitter wells, the use of magnetic ranging techniques may result in inaccurate results. Thus, recovery of conductors may prove difficult because the blind-drilled bores may be viewed as undrillable due to anti-collision rules.